It has long been recognized that spent coffee grounds, such as those from a countercurrent multistage extraction system, can be hydrolyzed, particularly at elevated temperatures in the presence of an acid catalyst, to provide additional useful coffee solids. For instance, U.S. Pat. No. 2,573,406 to Clough et al. discloses a process for producing a soluble coffee which involves hydrolyzing atmospherically extracted coffee grounds in 1% sulphuric acid at 100.degree. C. for about one hour, neutralizing and filtering the hydrolysate, combining the hydrolysate with atmospheric extract and subsequently drying the combined extract to provide the soluble coffee. Benner et al. disclose a similar process in U.S. Pat. No. 2,687,335, only phosphoric acid is used in place of the sulphuric acid. Both prior art methods are relatively slow and inefficient, requiring upwards of one hour to produce the hydrolysate. Moreover, neither the Clough et al. nor Benner et al. methods provide for the simultaneous recovery of the coffee oil known to be present in spent coffee grounds.
A more efficient hydrolysis method is disclosed in by Fulger et al. in U.S. Pat. No. 4,508,745, issued on Apr. 2, 1985. The commonly assigned patent application discloses relatively high-temperature, short-time, acid-catalyzed hydrolysis of coffee material in a reactor, typically a plug flow reactor. The coffee material is slurred in water, resulting in a relatively dilute hydrolysate, at times less than about 5% by weight coffee solids. Such a dilute hydrolysate can require extensive concentration, representing an additional cost. Moreover, Fulger et al. do not provide for the concurrent extraction of the coffee oil from the coffee material.
It is an object of the present invention to provide a method of simultaneously hydrolyzing spent coffee grounds and extracting the coffee oil therefrom.
Another object of the invention is to provide a method of hydrolyzing spent coffee grounds which produces a more concentrated hydrolysate.